The present invention generally relates to optical integrated circuits and more particularly to a waveguide-grating router (WGR), which is based on an arrayed-waveguide grating (AWG), associated with improving wavelength division multiplexing, including dense wavelength division multiplexing (DWDM).
As the amount of data traffic increases in optical networks, it becomes increasingly important to provide improved wavelength division multiplexing, demultiplexing and routing devices. One such device is a waveguide-grating router WGR that facilitates DWDM. DWDM allows multiple beams of light of different wavelengths carrying separate data channels to be transmitted along a single optical fiber. WGR devices can be employed to combine and/or separate optical signal carrying data channels coded in light beams with different wavelengths.
One technique for fabricating a waveguide-grating router is planar lightwave circuit (PLC) technology. A typical PLC comprises planar waveguides and/or channel waveguides. Examples of planar and channel waveguides are shown in H. Kogelnik, Theory of Optical Waveguides, Guided-Wave Optoelectonics T. Tamir ed., Springer-Verlag, Berlin, 1988, and also by H. Nishihara, M. Haruna, and T. Suhara, Optical Integrated Circuits, McGraw Hill, New York, 1987.
In a planar (or slab) waveguide, light is generally restricted to propagate in a region that is thin (typically between 3 xcexcm and 30 xcexcm) in one dimension, referred to herein as the lateral dimension or height, and extended (typically between 1 cm and 100 cm) in the other two dimensions. Herein, xe2x80x9cslab waveguidexe2x80x9d does not necessarily imply that the waveguide comprises layers of uniform refractive index, rather xe2x80x9cslab waveguidexe2x80x9d may refer to, but is not limited to, any type of planar waveguide, including graded index planar waveguides. Herein, we refer to the plane that is perpendicular to the lateral dimension of the PLC as the plane of the PLC. The longitudinal direction is defined to be the direction of propagation of light at any point on the PLC. Further, the lateral direction is defined to be perpendicular to the plane of the PLC and the transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
In a channel waveguide, light has an optical field that is substantially confined in both the lateral direction and the transverse direction. In a typical channel waveguide, the field is substantially confined within a region that extends between 3 xcexcm and 30 xcexcm in the lateral direction, herein referred to as the height, and extends between 3 xcexcm and 100 mm in the transverse direction, herein referred to as the width.
There are various approaches to building a PLC. In a typical example of a PLC, a slab waveguide comprises three layers of silica glass with the core layer lying between the top cladding layer and the bottom cladding layer. Channel waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. Further, each layer may be doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer. As a second example, slab waveguides and channel waveguides comprise three or more layers of InGaAsP and adjacent layers can have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the slab waveguide and/or channel waveguide may comprise an optically transparent polymer. A fourth example of a slab waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
In addition to the channel and slab waveguides described above, various PLCs may comprise at least one optical dispersive region such as, for example, an arrayed waveguide. Typically, a waveguide-grating router (WGR) is a planar lightwave circuit and comprises at least one input channel waveguide, an input slab waveguide, an arrayed-waveguide grating (AWG), an output slab waveguide, and at least one output channel waveguide. Herein, the term xe2x80x9cinput waveguidexe2x80x9d implies xe2x80x9cinput channel waveguidexe2x80x9d and xe2x80x9coutput waveguidexe2x80x9d implies xe2x80x9coutput channel waveguide;xe2x80x9d however, xe2x80x9cinput slab waveguidexe2x80x9d does not imply xe2x80x9cinput channel waveguidexe2x80x9d and xe2x80x9coutput slab waveguidexe2x80x9d does not imply xe2x80x9coutput channel waveguide.xe2x80x9d
The arrayed-waveguide grating comprises an array of waveguides. The length of the ith waveguide in the AWG is denoted as Li. The angular dispersion that is provided by the AWG is determined in part by the difference in length between adjacent waveguides, Li+1-Li. The details of construction and operation of the WGR are described in M. K. Smit and C. Van Dam, PHASAR-Based WDM-Devices: Principles, Design, and Application, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 2, pp. 236-250 (1996); K. McGreer, Arrayed Waveguide Gratings For Wavelength Routing, IEEE Communication Magazine, Vol. 36, No. 12, pp. 62-68 (1998); and K. Okamoto, Fundamentals of Optical Waveguides, pp. 346-381, Academic Press, San Diego, Calif., USA (2000). Each of the publications and patents referred to in this application is herein incorporated by reference in its entirety.
Such WGRs are measured by performance parameters like insertion loss, isolation, uniformity of output signal, number of channels and data throughput, for example. As with any filter, WGRs do not perform the wavelength selection involved in (de)multiplexing perfectly. Such imperfect selection can lead to reduced isolation. Isolation concerns the difference between the signal power and the unwanted noise in the passband. The number of channels depends, at least in part, on the transfer function associated with each channel.
The transfer function describes the optical coupling between a particular input waveguide and a particular output waveguide as a function wavelength of light; the spectral transmissivity (i.e. the spectrum) describes the optical power that is coupled between a particular input waveguide and a particular output waveguide as a function wavelength of light (or, equivalently, as a function of frequency of light); the passband refers to a peak region in the spectral transmissivity associated with a particular input waveguide and a particular output waveguide; and herein xe2x80x9cinsertion lossxe2x80x9d refers to the maximum value of transmissivity within the passband. Typically, the passband refers to the portion of the spectral transmissivity that is greater than about xe2x88x9220 dB below the insertion loss. Each passband is characterized by a central wavelength, a central frequency, and one or more values associated with the width of the passband. However, conventionally, the passbands associated with the light beams of different wavelengths may not have been consistent across the output waveguides, and thus, improved WGR operation is desired.
The term xe2x80x9cbandwidthxe2x80x9d refers to a parameter that characterizes the width of a passband; however, the term can be used in more than one way according to the context in which it is used or according to clarifying definitions imposed upon it for a particular context. Generally, bandwidth refers to the value of a wavelength range or a frequency range for which the transmissivity of a particular passband is greater than or equal to a particular reference level for all polarization states of light. Typical examples of reference levels are 0.5 dB, 1 dB and 3 dB below the maximum transmissivity of the particular passband. Herein, xe2x80x9cfrequency-limited bandwidthxe2x80x9d will refer to a value of bandwidth that is specified in frequency and will be denoted as xcex4v; and xe2x80x9cwavelength-limited bandwithxe2x80x9d will refer to a value of bandwidth that is specified in wavelength and will be denoted as xcex4xcex. xe2x80x9cFrequency-limited bandwidthxe2x80x9d and xe2x80x9cwavelength-limited bandwithxe2x80x9d will not imply any particular reference level; however, any suitable reference level may be used in association with either term as used herein.
In addition to the passband, the stopband affects the performance of a WGR. The stopband refers to the portion of the spectral transmissivity (which, again, is determined by the transfer function) that is not within the passband. The stopband affects, for example, the adjacent channel isolation. Adjacent channel isolation refers to the degree to which one output waveguide rejects light that is intended to be maximally coupled into an adjacent output waveguide. When the adjacent-channel isolation is determined over a range of wavelengths, it is referred to herein as the wavelength-limited adjacent isolation. When the adjacent-channel isolation is determined over a range of frequencies, it is referred to herein as the frequency-limited adjacent isolation.
One type of WGR is a Gaussian-passband WGR (G-WGR). In a G-WGR, the length difference between adjacent waveguides of the AWG, Li+1-Li, is substantially independent of i (i.e., Li+1-Li is substantially constant throughout the AWG.). This type of WGR is described in K. Okamoto, Fundamentals of Optical Waveguides, pp. 346-360, Academic Press, San Diego, Calif., USA (2000). The shape of the passband is determined by the convolution of two fields. The first field in the convolution is the field that is formed from the light that passes through the AWG and is imaged onto the output focal curve. The second field in the convolution is the fundamental mode of the output waveguide. In the G-WGR, both fields in the convolution are substantially Gaussian, and, consequently, the passband is substantially Gaussian.
Another type of WGR is a passband-flattened WGR (PF-WGR). The passband of the PF-WGR is typically broader than the passband of a G-WGR. In this context, a passband that is relatively broad refers to a passband having a value of flatness that is relatively large wherein flatness is defined as the xe2x88x921 dB bandwidth divided by the xe2x88x9220 dB bandwidth. Typically, a G-WGR has a passband flatness of approximately 0.22, and typically a PF-WGR is required to have a flatness of 0.3 or larger. A broad passband is advantageous for applications that require the passband to be broader than can be provided by the G-WGR. There are a variety of techniques to flatten the passband of an WGR. One technique for broadening the passband of a WGR involves the introduction of a parabolic taper (horn) between the slab waveguide and the channel waveguide at either the input side or the output side. An example of a PF-WGR optical router is disclosed in K. Okamoto and A. Sugita, Flat Spectral Response Array-Waveguide Grating Multiplexer With Parabolic Waveguide Horns, Electronics Letters, Vol. 32, No. 18, pp. 1661-1662 (1996).
WGRs attempt to provide substantial uniformity in passband throughout the output channels so that, for example, a first output channel carrying waves of a first wavelength has the same parameters (e.g., bandwidth, isolation, and insertion loss, etc.) as a second output channel carrying waves of a second wavelength. Since WGRs are commonly employed to multiplex and/or demultiplex channels, parameters associated with the passband are therefore important in determining the separation between channels and thus the number of channels available in such a (de)multiplexer. WGRs with improved characteristics are desired.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
Conventionally, the receiving ends of output waveguides that receive light from a WGR are the same width at the point where they are optically connected to an output slab waveguide. One aspect of the present invention concerns improving WGR performance by providing receiving ends of output waveguides that are of various widths to facilitate providing a transfer function that is optimized for each individual output waveguide. For example, various widths may be provided to facilitate improved uniformity of a figure of merit, which may include, but is not limited to, frequency-limited bandwidth, wavelength-limited bandwidth, adjacent isolation, or insertion loss.
In conventional WGRS, the delivering end of the output slab waveguide is not fashioned to account for improvements that can be achieved by positioning the receiving ends of the output waveguides at precise locations relative to an output focal curve. One aspect of the present invention concerns improving WGR performance by shaping the delivering end of the output slab waveguide to facilitate positioning the receiving ends of the output waveguides at desired locations relative to the output focal curve. For example, a delivering end may be provided to facilitate improved uniformity of a figure of merit, which may include, but is not limited to, frequency-limited bandwidth, wavelength-limited bandwidth, adjacent isolation, or insertion loss.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention may become apparent from the following detailed description of the invention when considered in conjunction with the drawings.